Defrost system

ABSTRACT

To provide a defrost system capable of preferable defrosting and prevention of generation of icicles on a casing without installing a brine circuit. A defrost system includes a thermosiphon defrost circuit that is provided by being branched from a circulation line, in which, at the time of defrosting, a CO2 refrigerant staying inside a fin-tube heat exchanger repeats a two-phase change of a gaseous form and reliquefaction, and which forms a CO2 circulation path together with the fin-tube heat exchanger; electromagnetic opening/closing valves and that are closed at the time of defrosting and set the CO2 circulation path to a closed circuit; and a first electric heater arranged above the thermosiphon defrost circuit so as to be adjacent to the thermosiphon defrost circuit, and naturally circulates the CO2 refrigerant in the closed circuit at the time of defrosting.

TECHNICAL FIELD

The present invention relates to a defrost system applied to a refrigeration apparatus that cools the interior of a cold storage room by circulating a CO₂ refrigerant in a cooler provided in the cold storage room, and for removing frost attached to a fin-tube heat exchanger provided in the cooler.

BACKGROUND ART

From the viewpoint of preventing ozone layer depletion, preventing global warming, and the like, a refrigeration apparatus in which, as a refrigerant of a refrigeration apparatus used for indoor air conditioning, refrigeration of food, and the like, ammonia that has high cooling performance but is toxic is used as a primary refrigerant and CO₂ that is non-toxic and odorless is used as a secondary refrigerant has widely been used.

In such a refrigeration apparatus, a primary refrigerant circuit in which ammonia refrigerant circulates and a secondary refrigerant circuit in which CO₂ refrigerant circulates are connected by a cascade condenser, and heat is transferred between the ammonia refrigerant and the CO₂ refrigerant in the cascade condenser. The CO₂ refrigerant cooled and liquefied by the ammonia refrigerant is sent to the cooler provided inside the cold storage room, and cools the air inside the cold storage room via the fin-tube heat exchanger provided inside the casing of the cooler. The CO₂ refrigerant partially vaporized by cooling the air in the cold storage room returns to a CO₂ receiver via the secondary refrigerant circuit and is again cooled and liquefied by the cascade condenser.

During operation of the refrigeration apparatus, frost forms on the heat exchange tube provided in the cooler and reduces the heat transfer efficiency, and therefore it is necessary to perform defrosting (frost removal).

In this regard, for example, in Patent Literature 1 below, a defrost system in which a defrost circuit (thermosiphon defrost circuit) and a warm brine circuit are installed and which includes a first heat exchanger for heating a CO₂ refrigerant circulating in the defrost circuit with warm brine is disclosed. With the defrost system configured as described above, a CO₂ refrigerant liquid in a closed circuit drops by gravity to the first heat exchanger in the defrost circuit, and is heated and vaporized by the warm brine in the first heat exchanger. The vaporized CO₂ refrigerant rises in the defrost circuit by the thermosiphon effect, and the risen CO₂ refrigerant gas heats and melts the frost attached to the outer surface of the fin-tube heat exchanger provided inside the cooler. The CO₂ refrigerant that is liquefied by heating the fin-tube heat exchanger descends in the defrost circuit by gravity. The CO₂ refrigerant liquid that has descended to the first heat exchanger is again heated and vaporized in the first heat exchanger.

CITATION LIST Patent Literatures

-   Patent Literature 1: JP 2015/093233 X

SUMMARY OF INVENTION Technical Problem

In the defrost system disclosed in Patent Literature 1, since the warm brine circuit is installed, a warm brine facility becomes bulky and the concentration control of the warm brine is required.

On the other hand, it is important to prevent icicles from forming in the lower part of the cooler by melting water flowing from the upper part of the cooler during defrosting.

The present invention was invented to solve the above problems, and an object is to provide a defrost system capable of preferable defrosting of a cooler and preventing icicles from being generated in a fin-tube heat exchanger at a lower part of a casing without having to install a warm brine circuit for heating a thermosiphon defrost circuit.

Solution to Problem

A defrost system according to the present invention for achieving the above object is a defrost system for a refrigeration apparatus in which a cooler having a casing, a fin-tube heat exchanger provided inside the casing, and a drain pan provided below the fin-tube heat exchanger is provided inside a cold storage room, including a circulation line that is connected to the fin-tube heat exchanger of the cooler and circulates a CO₂ refrigerant having a low temperature at the time of cooling, and a refrigeration cycle that cools and reliquefies the CO₂ refrigerant in a gaseous form with a refrigerant that circulates inside. The defrost system includes a thermosiphon defrost circuit that is provided by being branched from the circulation line, in which, at the time of defrosting, the CO₂ refrigerant staying inside the fin-tube heat exchanger repeats a two-phase change of a gaseous form and reliquefaction, and which forms a CO₂ circulation path together with the fin-tube heat exchanger; an opening/closing valve that is closed at the time of defrosting and sets the CO₂ circulation path to a closed circuit; and a first electric heater arranged above a thermosiphon defrost circuit so as to be adjacent to the thermosiphon defrost circuit, and naturally circulates the CO₂ refrigerant in the closed circuit at the time of defrosting.

With the defrost system configured as described above, the CO₂ refrigerant liquid in the closed circuit drops by gravity to the first electric heater in the thermosiphon defrost circuit, and is heated and vaporized by the first electric heater. The vaporized CO₂ refrigerant rises in the thermosiphon defrost circuit by the principle of thermosiphon, and the risen CO₂ refrigerant gas heats the fin-tube heat exchanger provided inside the cooler, and heats and melts the frost attached to the outer surface of the fin-tube heat exchanger. The CO₂ refrigerant that is liquefied by heating the fin-tube heat exchanger descends in the thermosiphon defrost circuit by gravity. The CO₂ refrigerant liquid that has descended to the first electric heater is heated and vaporized again by the first electric heater. From the above, it is possible to preferably perform defrosting of a cooler and prevent icicles from being generated in a fin-tube heat exchanger at a lower part of a casing without having to install a warm brine circuit for heating a thermosiphon defrost circuit.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an overall configuration diagram of a refrigeration apparatus according to the present embodiment.

FIG. 2 is a schematic perspective view of a cooler, a defrost system, and the like according to the present embodiment.

FIG. 3 is a schematic diagram of a cooler and a defrost system according to the present embodiment.

FIG. 4 is a sectional view taken along line 4-4 in FIG. 3.

FIG. 5 is a sectional view taken along line 5-5 in FIG. 3.

FIG. 6 is a schematic diagram showing a thermosiphon defrost circuit according to the present embodiment.

FIG. 7 is a diagram for explaining a circulation path of a CO₂ refrigerant at the time of defrosting.

FIG. 8A is a diagram showing a state in which an opening of a fan is close.

FIG. 8B is a diagram showing a state in which the opening of the fan is opened.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described with reference to FIGS. 1 to 6. Note that, in the description of the drawings, the same elements will be denoted by the same reference symbols, and redundant description will be omitted. The dimensional ratios in the drawings are exaggerated for the sake of convenience of description, and may differ from the actual ratios.

FIG. 1 is an overall configuration diagram of a refrigeration apparatus 1 according to the present embodiment. FIG. 2 is a schematic perspective view of a cooler 11, a defrost system 20, and the like according to the present embodiment. FIG. 3 is a schematic diagram of the cooler 11 and the defrost system 20 according to the present embodiment. FIG. 4 is a sectional view taken along line 4-4 in FIG. 3. FIG. 5 is a sectional view taken along line 5-5 in FIG. 3. FIG. 6 is a schematic diagram showing a thermosiphon defrost circuit 21 according to the present embodiment.

As shown in FIG. 1, a refrigeration apparatus 1 includes a pair of coolers 11 provided in a cold storage room 10, a defrost system 20 provided in the cooler 11, a circulation line (secondary refrigerant circuit) 30 through which a CO₂ refrigerant circulates, a CO₂ receiver 40 for storing the CO₂ refrigerant, an ammonia refrigeration cycle 50 (refrigeration cycle) including a circulation line (primary refrigerant circuit) 56 in which an ammonia refrigerant circulates, a cooling water circuit 60 in which cooling water circulates, and a closed cooling tower 70 connected to the cooling water circuit 60.

In the cold storage room 10, as shown in FIG. 1, two coolers 11 are provided vertically. Since the configurations of the two coolers 11 are the mutually same configuration, and therefore the configuration of one cooler 11 will be described here.

As shown in FIG. 1, the cooler 11 includes a casing 12, a fin-tube heat exchanger 13 provided inside the casing 12, and a fan 15 that forms an airflow that flows in and out of the casing 12.

As shown in FIG. 2, the casing 12 is configured in a substantially rectangular shape. Inside the casing 12, the fin-tube heat exchanger 13 is arranged. Further, a second electric heater 23 is arranged below the lowermost part of the fin-tube heat exchanger 13, and a third electric heater 24 is arranged below a dummy pipe L provided at the lowermost part of the casing 12. The second electric heater 23 and the third electric heater 24 constitute a lower electric heater. The dummy pipe L is provided to prevent bridges due to icicles of a drain pan 83 to be described below and a heat exchange tube 13A of the fin-tube heat exchanger 13 and to ensure a uniform front wind speed, and the CO₂ refrigerant does not circulate.

The fin-tube heat exchanger 13 includes the heat exchange tube 13A and fins 13B as shown in FIGS. 2 and 3. As shown in FIG. 3, the heat exchange tube 13A is formed in a meander shape in a vertical direction and in a horizontal direction inside the casing 12. The fin 13B is formed in the vertical direction as shown in FIG. 2. Further, as shown in FIG. 3, four heat exchange tubes 13A are provided along a depth direction of the casing 12. Note that the configuration of the heat exchange tube 13A is not limited thereto as long as it is evenly arranged inside the casing 12.

As shown in FIG. 3, the four heat exchange tubes 13A are coupled to an inlet header 16 at a lower end of the four heat exchange tubes 13A. Further, as shown in FIG. 3, the four heat exchange tubes 13A are coupled to an outlet header 17 at an upper end of the four heat exchange tubes 13A.

The fan 15 is arranged above the casing 12 as shown in FIG. 1. Note that the position where the fan 15 is provided may be a side surface of the casing 12, or the like. When the fan 15 operates, an airflow that flows in and out of the casing 12 is formed.

The defrost system 20 is provided for melting and removing (defrosting) frost attached to a surface of the fin-tube heat exchanger 13. As shown in FIGS. 1 to 5, the defrost system 20 includes the thermosiphon defrost circuit 21, a first electric heater 22, the second electric heater 23, and the third electric heater 24.

As shown in FIG. 1, the thermosiphon defrost circuit 21 is provided by being branched from a CO₂ feed line 31 of the circulation line 30, and forms a CO₂ circulation path together with the fin-tube heat exchanger 13. Further, a heat collection portion of the thermosiphon defrost circuit 21 is arranged below the first electric heater 22.

As shown in FIGS. 1 and 3, in the thermosiphon defrost circuit 21, an electromagnetic opening/closing valve 21A and a check valve 21J are arranged. At the time of defrosting, the thermosiphon defrost circuit 21 closes electromagnetic opening/closing valves 34A and 34B, which will be described below, and opens the electromagnetic opening/closing valve 21A to form the CO₂ circulation path through which CO₂ circulates. On the other hand, the thermosiphon defrost circuit 21 opens the electromagnetic opening/closing valves 34A and 34B and closes the electromagnetic opening/closing valve 21A at the time of a refrigerating operation.

The configuration of the thermosiphon defrost circuit 21 will be described below in detail with reference to FIGS. 3 and 6.

As shown in FIGS. 3 and 6, the thermosiphon defrost circuit 21 includes a first line 21B branched from the CO₂ feed line 31 of the circulation line 30, a first header 21C to which an end of the first line 21B is connected, three second lines 21D, 21E, 21F extending from the first header 21C, a second header 21G to which the three second lines 21D, 21E, 21F are coupled and which is provided at a position higher than the first header 21C, and a third line 21H extending from the second header 21G and connected to a CO₂ return line 32 of the circulation line 30.

The three second lines 21D, 21E, 21F, as shown in FIG. 6, include the second line 21D connecting the most distant parts of the first header 21C and the second header 21G in a meander shape, the second line 21E connecting the closest parts of the first header 21C and the second header 21G in a meander shape, and the second line 21F arranged between the second line 21D and the second line 21E. With this configuration, the three second lines 21D, 21E, 21F are arranged in an upward inclination without crossing each other, and therefore, in the three second lines 21D, 21E, 21F, CO₂ can be circulated preferably.

As shown in FIGS. 1, 2, and 5, the first electric heater 22 is arranged below the drain pan 83 described below and above the three second lines 21D, 21E, 21F. As shown in FIG. 2, the first electric heater 22 is configured such that six heaters have a U shape. The output per heater is not particularly limited, but is 1.5 kW.

The second electric heater 23 is, as shown in FIGS. 1, 2, and 5, arranged below the fin-tube heat exchanger 13 inside the casing 12. Specifically, as shown in FIG. 5, the second electric heater 23 is arranged below the heat exchange tube 13A and above the dummy pipe L. The output of one heater is not particularly limited, but is 1.5 kW. Since the second electric heater 23 is arranged below the fin-tube heat exchanger 13 inside the casing as described above, water droplets descending the fin-tube heat exchanger 13 can be recovered by the drain pan 83 without being refrozen to be icicles at the fin-tube heat exchanger 13 at a lower part of the casing 12.

The third electric heater 24 is, as shown in FIG. 5, arranged below the dummy pipe L. That is, the third electric heater 24 is arranged at the lowermost part inside the casing 12. Since the third electric heater 24 is arranged at the lowermost part inside the casing 12, it is possible to preferably prevent refreezing at a lower part of the casing 12 to generate icicles.

As shown in FIGS. 2 and 5, a heat insulating material 81 is provided below the thermosiphon defrost circuit 21. The thickness of the heat insulating material 81 is not particularly limited, but is, for example, 20 mm, and prevents heat radiation loss from the lower surface of the thermosiphon defrost circuit 21 heated by the first electric heater 22. The drain pan 83 is provided above the first electric heater 22, and water droplets at the time of defrosting can be drained from a drain discharge pipe 83A without refreezing. Further, between the thermosiphon defrost circuit 21 and the first electric heater 22, a heat transfer plate 82 is provided. By providing the heat transfer plate 82 in this way, the heat of the first electric heater 22 can be appropriately transferred to the heating of the CO₂ refrigerant.

The circulation line 30 is configured to circulate the CO₂ refrigerant. The circulation line 30, as shown in FIG. 1, includes the CO₂ feed line 31 for feeding the CO₂ refrigerant in a liquid form to the pair of cold storage rooms 10 from the CO₂ receiver 40, the CO₂ return line 32 for returning a gas-liquid mixed CO₂ refrigerant coming out of the pair of cold storage rooms 10 to the CO₂ receiver 40, and a reliquefaction line 33 for reliquefying the gasified CO₂ refrigerant.

The CO₂ feed line 31 is, as shown in FIG. 1, connected to a lower part of the CO₂ receiver 40. Further, the CO₂ return line 32 is, as shown in FIG. 1, connected to an upper part of the CO₂ receiver 40.

Further, a first pump P1 is provided in the CO₂ feed line 31, and the CO₂ refrigerant in a liquid form in the CO₂ receiver 40 is fed to the cooler 11 in the cold storage room 10 by the first pump P1.

As shown in FIG. 1, the CO₂ feed line 31 is branched into a first feed line 31A connected to one cooler 11 and a second feed line 31B connected to the other cooler 11.

The first feed line 31A is connected to a first return line 32A via the one cooler 11. Further, the second feed line 31B is connected to a second return line 32B via the other cooler 11. The first return line 32A and the second return line 32B join again and are coupled to the CO₂ return line 32.

The first feed line 31A is, as shown in FIGS. 1 and 3, connected to the inlet header 16, and the first return line 32A is connected to the outlet header 17. As shown in FIG. 1, an electromagnetic opening/closing valve (opening/closing valve) 34A is arranged in the first feed line 31A, and an electromagnetic opening/closing valve (opening/closing valve) 34B is arranged in the first return line 32A.

As shown in FIG. 1, a pressure sensor 34 is connected to the first return line 32A. A control portion 35 to which a detection value of the pressure sensor 34 is input is connected to the pressure sensor 34. Further, a controller 36 of the first electric heater 22 is connected to the control portion 35, and the control portion 35 can control the temperature of the first electric heater 22 and ON/OFF of the six heaters.

At the time of defrosting, the control portion 35 can reduce the temperature of the first electric heater 22 or reduce the number of heaters of the first electric heater 22 among the six heaters to be turned on when the pressure of the CO₂ circulation path measured by the pressure sensor 34 is higher than a predetermined pressure.

Further, the first return line 32A is provided with a branch circuit 37 that branches from the first return line 32A, the branch circuit 37 is provided with a pressure adjusting valve 38, and when the pressure is higher than a predetermined pressure, the pressure adjusting valve 38 is opened to reduce the pressure.

The reliquefaction line 33 is connected an upper part of the CO₂ receiver 40. When passing through the reliquefaction line 33, the CO₂ refrigerant in a gaseous form in the CO₂ receiver 40 is reliquefied by a heat exchanger 51 of the ammonia refrigeration cycle 50 described below. Then, the reliquefied CO₂ refrigerant in a liquid form returns to the CO₂ receiver 40.

The ammonia refrigeration cycle 50 circulates the ammonia refrigerant. The ammonia refrigeration cycle 50 cools and liquefies the CO₂ refrigerant in a gaseous form. As shown in FIG. 1, the ammonia refrigeration cycle 50 includes the heat exchanger (cascade condenser) 51 as an evaporator, a refrigeration compressor 52, a condenser 53, an ammonia receiver 54, an expansion valve 55, and the circulation line (primary refrigerant circuit) 56 through which the ammonia refrigerant circulates.

The ammonia refrigerant gas evaporated by the heat of the CO₂ refrigerant in a gaseous form in the heat exchanger 51 is compressed by the refrigeration compressor 52, the high temperature and high pressure ammonia refrigerant gas is cooled and condensed in the condenser 53, the liquefied ammonia refrigerant liquid is stored in the ammonia receiver 54, the ammonia refrigerant liquid in the ammonia receiver 54 is fed to and expanded by the expansion valve 55, and the low-pressure ammonia refrigerant liquid is fed to the heat exchanger 51 and is used for cooling CO₂ refrigerant in a gaseous form.

The cooling water circuit 60 is installed on the condenser 53. The cooling water circulating in the cooling water circuit 60 is heated by the ammonia refrigerant in the condenser 53.

The cooling water circuit 60 is connected to the closed cooling tower 70. The cooling water is circulated in the cooling water circuit 60 by a cooling water pump 61. The cooling water that has absorbed the exhaust heat of the ammonia refrigerant in the condenser comes into contact with the outside air and spray water in the closed cooling tower 70, and is cooled by the latent heat of vaporization of the spray water.

The closed cooling tower 70 includes a cooling coil 71 connected to the cooling water circuit 60, a fan 72 for ventilating outside air a through the cooling coil 71, a sprinkling pipe 73 and a pump 74 for spraying the cooling water on the cooling coil 71. A part of the cooling water sprayed from the sprinkling pipe 73 evaporates, and the latent heat of vaporization is used to cool the cooling water flowing through the cooling coil 71.

The configuration of the refrigeration apparatus 1 has been described heretofore. Next, with reference to FIGS. 1, 7, and 8, a method of using the refrigeration apparatus 1 according to the present embodiment will be described separately for the refrigerating operation and the defrosting.

FIG. 1 is a diagram showing a circulation path of a CO₂ refrigerant at the time of a refrigerating operation. At the time of the refrigerating operation, the electromagnetic opening/closing valves 34A and 34B are opened and the electromagnetic opening/closing valve 21A is closed. Thus, the CO₂ refrigerant supplied from the CO₂ feed line 31 circulates through the first feed line 31A, the second feed line 31B, and the fin-tube heat exchanger 13. On the other hand, by the operation of the fan 15 inside the cold storage room 10, a circulating flow of the inside air passing through the inside of the cooler 11 is formed. The inside air is cooled by the CO₂ refrigerant circulating through the fin-tube heat exchanger 13, and the inside of the cold storage room 10 is kept at a low temperature of −25° C., for example. At the time of the refrigerating operation, as shown in FIG. 8B, the fan 15 is operated to open a sock duct.

FIG. 7 is a diagram showing a circulation path of a CO₂ refrigerant at the time of defrosting. At the time of defrosting, the electromagnetic opening/closing valves 34A and 34B are closed and the electromagnetic opening/closing valve 21A is opened. This forms a closed CO₂ circulation path including the fin-tube heat exchanger 13 and the thermosiphon defrost circuit 21.

The CO₂ refrigerant liquid in the closed circuit drops by gravity in the thermosiphon defrost circuit 21 to the first header 21C and the three second lines 21D, 21E, 21F extending from the first header 21C, is heated and vaporized by the first electric heater 22. The vaporized CO₂ refrigerant rises in the check valve 21J of the thermosiphon defrost circuit 21 by the principle of thermosiphon, and the risen CO₂ refrigerant gas heats and melts the frost attached to the outer surface of the fin-tube heat exchanger 13 provided inside the cooler 11. The CO₂ refrigerant that is liquefied by heating the fin-tube heat exchanger 13 descends in the thermosiphon defrost circuit 21 by gravity. The CO₂ refrigerant liquid that has descended to the first header 21C and the three second lines 21D, 21E, 21F extending from the first header 21C is again heated and vaporized by the first electric heater 22.

The melt water obtained as the frost is heated and melted falls toward the drain pan 83. At this time, for example, with the configuration in which the second electric heater 23 is not provided, there is a possibility that icicles are formed as refreezing occurs below the fin-tube heat exchanger 13. On the other hand, with the defrost system 20 according to the present embodiment, since the second electric heater 23 and the third electric heater 24 are provided at the lowermost part inside the casing 12, it is possible to prevent icicles from being formed below the casing 12. Further, at the time of defrosting, as shown in FIG. 8A, an opening of the fan 15 is closed by the sock duct to assist the temperature rise in the cooler 11 and prevent the generation of fog in the cold storage room 10. Note that the configuration in which the second electric heater 23 is not provided is also included in the present invention.

As described above, in the defrost system 20 of the refrigeration apparatus 1 according to the present embodiment, the cooler 11 including the casing 12, the fin-tube heat exchanger 13 provided inside the casing 12, and the drain pan 83 provided below the fin-tube heat exchanger 13 is provided inside the cold storage room 10. It is the defrost system 20 of the refrigeration apparatus 1 including the circulation line (secondary refrigerant circuit) 30 connected to the fin-tube heat exchanger 13 of the cooler 11 and in which a low-temperature CO₂ refrigerant circulates at the time of cooling, and the refrigeration cycle 50 that cools and reliquefies the CO₂ refrigerant in a gaseous form with a refrigerant circulating inside.

The defrost system 20 includes the thermosiphon defrost circuit 21 that is provided by being branched from the circulation line 30, in which, at the time of defrosting, the CO₂ refrigerant staying inside the fin-tube heat exchanger 13 repeats a two-phase change of a gaseous form and reliquefaction, and which forms a CO₂ circulation path together with the fin-tube heat exchanger 13; the opening/closing valves 34A and 34B that are closed at the time of defrosting and sets the CO₂ circulation path to a closed circuit; and the first electric heater 22 arranged above the thermosiphon defrost circuit 21 so as to be adjacent to the thermosiphon defrost circuit 21.

The CO₂ refrigerant is naturally circulated in the closed circuit at the time of defrosting. With the defrost system 20 configured in this way, the CO₂ refrigerant liquid in the closed circuit is heated and vaporized by the first electric heater 22, and rises in the thermosiphon defrost circuit 21 by the principle of thermosiphon, the risen CO₂ refrigerant gas heats the fin-tube heat exchanger 13 provided inside the cooler 11, and heats and melts the frost attached to the outer surface of the fin-tube heat exchanger 13. The CO₂ refrigerant that is liquefied by heating the fin-tube heat exchanger 13 descends in the thermosiphon defrost circuit 21 by gravity. The CO₂ refrigerant liquid that has descended to the first electric heater 22 is heated and vaporized by the first electric heater 22. Further, since the second electric heater 23 is provided at a lower part inside the casing 12, water droplets descending the fin-tube heat exchanger 13 can be recovered in the drain pan 83 without being refrozen to be icicles in the fin-tube heat exchanger 13 at a lower part of the casing 12. From the above, it is possible to preferably perform defrosting without installing a brine circuit, and it is possible to prevent the generation of icicles on the heat exchange tubes 13A and the fins 13B at a lower part of the casing 12.

Further, it includes the pressure sensor 34 for measuring the pressure of the CO₂ circulation path at the time of defrosting, and the control portion 35 that controls the first electric heater 22 such that the pressure of the CO₂ circulation path decreases when the measurement value measured by the pressure sensor 34 is higher than a predetermined pressure. With the defrost system 20 configured in this way, it is possible to prevent the pressure inside the thermosiphon defrost circuit 21 and the fin-tube heat exchanger 13 from becoming extremely high at the time of defrosting, and therefore it is possible to preferably prevent damage to the pipes of the thermosiphon defrost circuit 21 and the fin-tube heat exchanger 13.

Further, the thermosiphon defrost circuit 21 includes the first line 21B branched from the CO₂ feed line 31 of the circulation line 30 of the CO₂ refrigerant, the first header 21C to which an end of the first line 21B is connected, the three second lines 21D, 21E, 21F extending from the first header 21C, the second header 21G to which the three second lines 21D, 21E, 21F are connected and which is provided at a position higher than the first header 21C, and the third line 21H extending from the second header 21G and connected to the CO₂ return line 32 of the circulation line 30.

The three second lines 21D, 21E, 21F include the second line 21D connecting the most distant parts of the first header 21C and the second header 21G in a meander shape, the second line 21E connecting the closest parts of the first header 21C and the second header 21G in a meander shape, and the second line 21F arranged between the second line 21D and the second line 21E. With this configuration, because the three second lines 21D, 21E, 21F, which are arranged without crossing one another, can be preferably heated by the first electric heater 22 via the heat transfer plate 82, the CO₂ refrigerant can be naturally circulated.

With the defrost system 20 configured in this way, at the time of defrosting, it can be performed only by the first electric heater 22 that heats and naturally circulates the CO₂ refrigerant remaining in the pipes of the thermosiphon defrost circuit 21 and the fin-tube heat exchanger 13 and enables heating and draining of the drain pan 83 and the second electric heater 23 for preventing re-freezing at the fin-tube heat exchanger 13 at a lower part of the casing 12 (the third electric heater 24 if the dummy pipe L is present), and therefore it is possible to perform defrosting with very little electric power as compared with heater defrost in which heaters are evenly arranged in the arrangement of the fin-tube heat exchanger 13. Further, since the fin-tube heat exchanger 13 is directly heated, the delay in starting the defrosting can be eliminated.

Further, it further includes the branch circuit 37 provided by being branched from the circulation line 30, and on the branch circuit 37, the pressure adjusting valve 38 for reducing the pressure when the pressure in the circulation line 30 is higher than a predetermined pressure is arranged. With the defrost system 20 configured in this way, it is possible to prevent the pressure inside the thermosiphon defrost circuit 21 and the fin-tube heat exchanger 13 from becoming extremely high at the time of defrosting operation, and therefore it is possible to preferably prevent damage to the thermosiphon defrost circuit 21 and the fin-tube heat exchanger 13.

It should be noted that the present invention is not limited to the above-described embodiment, but can be variously modified within the scope of the claims.

For example, in the above-described embodiment, the thermosiphon defrost circuit 21 includes the first line 21B branched from the circulation line 30, the first header 21C to which an end of the first line 21B is connected, the three second lines 21D, 21E, 21F extending from the first header 21C, the second header 21G to which the three second lines 21D, 21E, 21F are coupled, and the third line 21H extending from the second header 21G and connected to the circulation line 30, but is not particularly limited as long as it is configured to form the CO₂ circulation path together with the fin-tube heat exchanger 13.

Further, in the above-described embodiment, the three second lines 21D, 21E, 21F are provided, but two or more may be provided.

Further, in the above-described embodiment, ammonia is used as the refrigerant of the refrigeration cycle, but it is not limited thereto, but chlorofluorocarbon or other natural refrigerants may be used.

Further, in the above-described embodiment, the two coolers 11 are provided, but one or three or more coolers 11 may be provided.

REFERENCE SIGNS LIST

-   1 refrigeration apparatus, -   10 cold storage room, -   11 cooler, -   12 casing, -   13 fin-tube heat exchanger, -   13A heat exchange tube, -   13B fin, -   20 defrost system, -   21 thermosiphon defrost circuit, -   21A electromagnetic opening/closing valve, -   21B first line, -   21C first header, -   21D, 21E, 21F second line, -   21G second header, -   21H third line, -   21J check valve -   22 first electric heater, -   23 second electric heater, -   30 circulation line, -   34 pressure sensor, -   34A, 34B electromagnetic opening/closing valve, -   35 control portion, -   37 branch circuit, -   38 pressure adjusting valve, -   83 drain pan. 

1. A defrost system for a refrigeration apparatus in which a cooler having a casing, a fin-tube heat exchanger provided inside the casing, and a drain pan provided below the fin-tube heat exchanger is provided inside a cold storage room, comprising: a circulation line that is connected to the fin-tube heat exchanger of the cooler and circulates a CO2 refrigerant having a low temperature at a time of cooling; and a refrigeration cycle that cools and reliquefies the CO2 refrigerant in a gaseous form with a refrigerant that circulates inside, the defrost system including: a thermosiphon defrost circuit that is provided by being branched from the circulation line, in which, at a time of defrosting, the CO2 refrigerant staying inside the fin-tube heat exchanger repeats a two-phase change of a gaseous form and reliquefaction, and which forms a CO2 circulation path together with the fin-tube heat exchanger; an opening/closing valve that is closed at the time of defrosting and sets the CO2 circulation path to a closed circuit; and a first electric heater arranged above the thermosiphon defrost circuit so as to be adjacent to the thermosiphon defrost circuit, and naturally circulating the CO2 refrigerant in the closed circuit at the time of defrosting.
 2. The defrost system for the refrigeration apparatus according to claim 1, comprising: a pressure sensor that measures a pressure of the CO2 circulation path at the time of defrosting; and a control portion that controls the first electric heater so that the pressure of the CO2 circulation path decreases when a measurement value measured by the pressure sensor is higher than a predetermined pressure.
 3. The defrost system for the refrigeration apparatus according to claim 1, further comprising a lower electric heater arranged at a lower part inside the casing.
 4. The defrost system for the refrigeration apparatus according to claim 1, wherein the thermosiphon defrost circuit includes: a first line branched from the circulation line of the CO2 refrigerant; a first header to which an end of the first line is connected; a plurality of second lines that extends from the first header, a second header to which the plurality of second lines is connected and which is provided at a position higher than the first header; and a third line that extends from the second header and is connected to the circulation line, and the plurality of second lines at least includes a line connecting most distant parts of the first header and the second header in a meander shape, and a line connecting closest parts of the first header and the second header in a meander shape.
 5. The defrost system for the refrigeration apparatus according to claim 1, further comprising a branch circuit provided by being branched from the circulation line, wherein on the branch circuit, a pressure adjusting valve that reduces a pressure when the pressure in the circulation line is higher than a predetermined pressure is arranged. 